Illumination apparatus with polarizing elements for beam shaping

ABSTRACT

Illumination apparatus includes a light source which directs a polarized source beam in a downstream direction along an optical axis and desirably focuses the beam. A polarization-altering element downstream from the source alters the polarization in a portion or portions of the source beam, so that the altered beam includes portions having different polarization directions. The altered beam passes downstream through a polarization-selective filter. The output beam from the polarization-selective filter can include either portion depending on the orientation of the filter. The arrangement can be used to provide a spot of a given size and shape at the focal location in a scanner such as a bar code scanner. The size and depth of field of the spot can be varied dynamically during operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. ProvisionalApplication No. 60/173,532 filed Dec. 29, 1999, the disclosure of whichis hereby incorporated herein.

BACKGROUND OF THE INVENTION

Bar code scanners and other optical devices use focused beams of light.For example, in one type of bar code scanner, a beam of light from alaser diode is projected along an optical axis and focused to a spot oflight at a focal location in a working region along the axis. A movingmirror positioned in the path of the beam sweeps the spot along a lineof motion within a working region. A photodetector detects lightreflected from the working region and converts the reflected light to anelectrical signal. If an object bearing a pattern of light and darkregions such as a typical bar code is placed in the working region sothat the moving spot sweeps across these regions, the reflected lightwill vary in a pattern corresponding to the pattern of light and darkregions. The electrical signal will vary in the same manner. Theelectrical signal can be detected and decoded to yield the informationstored in the bar code.

The size and shape of the spot of light will affect the scanningoperation. Typically, the light which forms the spot has non-uniformintensity, which decreases gradually adjacent the periphery of the spot.The area in which the intensity exceeds a selected value, such as aselected proportion of the maximum intensity, is regarded as the spot.If the spot is too large in relation to the light and dark features tobe detected, the light will illuminate both light and dark featuressimultaneously with substantial intensity, and hence it will beimpossible to read the bar code properly. In many cases, a non-circularspot is desirable. Typically, the spot has a long dimension transverseto the direction of motion of the spot and a short dimension in thedirection of motion. For example, this arrangement can be used inscanners which are intended to read a conventional one-dimensional barcode consisting of a pattern of light and dark strips extending parallelto one another.

In some bar code scanning applicatons, the distance between the scannerand the object bearing the code may vary from object to object. Forthese applications, it is important to provide a light beam with asubstantial depth of field, i.e., a beam which has relatively smalldimensions throughout a substantial range of locations around thenominal focal location. In other applications, it is more important toassure that the beam focuses to the minimum breadth at the nominal focallocation, to provide the smallest beam “waist” and hence provide thesmallest spot size at the nominal focal location.

Typical light source apparatus used heretofore has incorporated a laserdiode, a collimating lens for focusing the light from the diode to aspot at a focal location, and an opaque plate with an aperture disposedbetween the lens and the focal spot for blocking light at the peripheryof the beam. The characteristics of the beam depend upon thecharacteristics of the aperture. For example, a non-circular aperturemay be used to form a non-circular beam. A large aperture yields a beamwith a low “f-number” which is sharply focused to a small size at thenominal focal location but which increases in size rapidly with distancefrom the nominal focal location, i.e., a beam with good resolution butpoor depth of field A narrow aperture yields a beam with a high f-numberwhich has a somewhat larger spot size at the nominal focal location butwhich increases in size more slowly with distance from the nominal focallocation. Such a beam has relatively poor resolution but good depth offield. Systems of this type are disclosed, for example, in U.S. Pat.Nos. 4,816,660 and 5,247,162.

There has been a need heretofore for further improvements in light beamsources and in scanning apparatus incorporating the same.

SUMMARY OF THE INVENTION

The present invention addresses these needs.

One aspect of the invention provides illumination apparatus for formingan output light beam. The apparatus according to this aspect of theinvention desirably includes a light source adapted to emit a sourcebeam of polarized light in a downstream direction along an optical axis,the source beam having a polarization direction transverse to the axis.The apparatus also preferably includes one or more polarization-alteringelements disposed downstream from said source. The one or morepolarization-altering elements are operative to alter the polarizationof the source beam nonuniformly so as to form an altered beam having afirst portion with a first polarization direction and a second portionhaving a second polarization direction different from the firstpolarization direction. Preferably, the second polarization direction isperpendicular to the first polarization direction. The first portion ofthe altered beam has a first intensity distribution relative to theoptical axis, and the second portion has a second intensity distributionrelative to this axis which preferably is different from the firstintensity distribution. For example, the polarization-altering elementmay be a birefringent element having a hole aligned with the opticalaxis. Light passing through the hole constitutes the first portion ofthe beam, and has a relatively narrow intensity distribution withmaximum intensity near the axis and substantially excluding lightoutside of a central zone close to said axis. Light passing through theregion of the birefringent element which surrounds the hole forms thesecond portion of the beam and has a second intensity distribution whichincludes substantial light outside of the central zone.

The apparatus desirably includes one or more polarization-selectiveelements disposed along the optical axis downstream from the one or morepolarization-altering elements. Each polarization-selective element hasa transmission axis and is operative to allow transmission of lighthaving a polarization direction parallel to the transmission axis ofsuch element and to block transmission of light having a polarizationdirection perpendicular to the transmission axis of such element. Theone or more polarization-selective elements may include a singlepolarizer. If the transmission axis of the polarizer is aligned with thepolarization direction of the first portion of the altered beam, anoutput beam passing downstream along the optical axis from the polarizerwill consist essentially of the light in the first portion of the beam.In the example given above, where the intensity distribution of thefirst portion is narrow, the output beam will form a spot having a higheffective f-number. Such a beam will resemble the beam formed by a smallaperture; it will have a relatively large spot size at the nominalfocus, but will have a relatively large depth of field, so that the spotsize increases slowly with distance from the nominal focus.

One or more of the elements in the system, such as the light source, thepolarization-altering element, and the polarization-selective elementmay be movable or otherwise adjustable so as to vary the effect of theseelements during operation, and thus change the configuration of theoutput beam during operation. Thus, the effective f-number of the outputbeam will vary dynamically. For example, in a system where thepolarization-selective element has a transmission axis parallel to thepolarization direction of the source beam, the polarization-alteringelement can be temporarily disabled. In this condition, the output beamwill include essentially all of the light in the square beam. The outputbeam will form a spot having a low effective f-number. The spot sizewill be relatively small at the nominal focus of the beam, but willincrease rapidly with distance from the nominal focus. When thepolarization-altering element is enabled, the beam returns to a higheffective f-number.

A further aspect of the invention provides scanners incorporatingillumination apparatus as discussed above. A scanner according to thisaspect of the invention desirably includes a frame and an illuminationapparatus as discussed above which is mounted to the frame so that theoutput beam from the illumination apparatus will be directed from theframe into a working region. The scanner desirably also includes aphotodetector for receiving light returned from the working region andproducing a signal representing the amplitude of the returned light. Thescanner may include means such as a movable or variable optical elementfor moving the output beam relative to the frame.

Yet another aspect of the invention provides methods of forming anoutput light beam. The method according to this aspect of the inventiondesirably includes the steps of directing a source beam of polarizedlight in a downstream direction along an axis and nonuniformly alteringthe polarization direction of the source beam to provide an altered beamhaving first and second portions with different polarization directionsand with different intensity distributions relative to the axis. Mostpreferably, the method includes the step of passing the altered beamthrough a polarization-selective element having a transmission axis soas to allow light having a polarization direction matching thetransmission axis of such element to pass downstream into an outputbeam, while substantially excluding light with a polarization directionperpendicular to the transmission axis from the output beam. Methodsaccording to this aspect of the invention can be used to form outputbeams having the characteristics discussed above in connection with theapparatus.

An additional aspect of the invention provides methods of scanninginformation-bearing elements as, for example, bar-coded objects. Themethod according to this aspect of the invention desirably includes thesteps of providing a scanning beam focused to a spot at a focallocation; moving the scanning beam relative to the information-bearingelements and detecting light reflected from the information-bearingelements to provide a signal representing the information carried bysuch elements. Most preferably, the method further includes the step ofrepeatedly varying the spot size and depth of field of the scanning beamduring the moving and detecting steps so as to vary the beamcharacteristics between a first condition in which the beam has a smallspot size at the focal location but a small depth of field and a secondcondition in which the beam has a larger spot size at said focallocation but a larger depth of field. The varying step desirably isperformed so that each individual information-bearing element will bescanned by the beam in both of said conditions. Some information-bearingelements will be read best by the beam in the first condition, whereasother information-bearing elements will be read by the beam in thesecond condition. As further discussed below, preferred methods inaccordance with this aspect of the invention can provide scanningperformance superior to that achievable with a scanning beam of fixedconfiguration. The step of varying the beam configuration can beperformed using apparatus and methods according to the foregoing aspectsof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view depicting illumination apparatus accordingto one embodiment of the invention.

FIG. 2 is a graph depicting a relationship between spot size andposition in the apparatus of FIG. 1.

FIG. 3 is a diagrammatic view of a scanner according to a furtherembodiment of the invention.

FIG. 4 is a diagrammatic perspective view depicting illuminationapparatus according to a further embodiment of the invention.

FIG. 5 is a diagrammatic perspective view depicting illuminationapparatus according to yet another embodiment of the invention.

FIG. 6 is a fragmentary sectional view of a component used in apparatusaccording to yet another embodiment of the invention.

FIG. 7 is a diagrammatic perspective view depicting apparatus accordingto yet another embodiment of the invention.

FIG. 8 is a diagrammatic view depicting apparatus according to a stillfurther embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Apparatus in accordance with one embodiement of the invention includes alight source 10 in the form of a visible laser diode or “VLD” 11 with alens 14. The light source or VLD/lens assembly 10 produces a source beamof light. The light source projects this beam in a downstream direction(to the right as seen in FIG. 1) along an optical axis 12 and focusesthe source beam to a spot at a focal location 18 along the optical axis.The light from VLD 10 is inherently plane polarized, so that the sourcebeam 16 is plane polarized with a polarization direction transverse tothe optical axis.

The term “polarization direction” as used in this disclosure withreference to a beam of light or with reference to a portion of a beamshould be understood in its ordinary sense, as referring to the averagedirection of polarization of the light constituting the beam or portionof the beam. All light is constituted by individual waves or photonshaving individual electric and magnetic vectors transverse to thedirection of propagation of the light. In unpolarized light, thesevectors are oriented in random directions transverse to the direction ofpropagation, so that the vector sum of the electric vectors over anyappreciable time is zero and the light as a whole has no definablepolarization direction. In plane polarized light, also referred to aslinearly polarized light, the electric vectors of an appreciable portionof the waves constituting the light are aligned with a polarization axistransverse to the direction of propagation of the light and hencetransverse to the optical axis. The direction of this polarization axis,or another direction having a fixed relation to this direction, iscommonly referred to as the direction of polarization of the light.

Downstream from the light source 10 is polarization-altering element 20in the form of a wave retarder or birefringent element which has acentral region 22 surrounding optical axis 12 removed from it so thatthere is a hole in the retarder. In the embodiment of FIG. 1, thepolarization-altering element 20 is disposed close to the lens 14 ofsource 10. Merely by way of example, the birefringent element mayinclude a polyvinyl alcohol film, laminated between a pair of celluloseacetate butyrate sheets. A first portion of the light in source beam 16passes through the hole and hence its polarization is unaffected. Asecond portion of the light passes through the retarder 20 and hence itspolarization is rotated away from the original polarization direction ofthe source beam. In effect, one portion of the light has itspolarization rotated with respect to the remaining portion of light.This forms an altered beam 24 having a first portion 23 with onepolarization direction and a second portion 25 with another polarizationdirection. The first portion 23, consisting of light passing throughhole 22, has the original polarization direction of the source beam andhas an intensity distribution concentrated close to optical axis 12. Thesecond portion 25, consisting of light passing through retarder 20outside of hole 22, has a different polarization direction and also hasa different intensity distibution; this portion of the beam has anintensity distribution spread over a region remote from axis 12. Becausethe altered beam has two portions with different polarizations, thelight can be manipulated further downstream. However, the altered beamcontains all or essentially all of the light in the source beam.

The two portions of the altered beam can have light that iscross-polarized. This can be accomplished by using a wave retarder 20known as a half wave plate, effective to rotate the polarization of thelight forming the second portion 90 degrees. A half-wave plate is madefrom a birefringent material that has the proper thickness for theparticular wavelength of light provided by source 10. In addition, torotate linearly polarized light 90 degrees, the fast axis of thehalf-wave plate must be at 45 degrees to the polarization direction ofthe light. If the fast axis of the retarder is not at a 45 degreeorientation to the polarization axis or does not have the requiredthickness, the two portions of the altered beam 24 will have light thatis not fully cross-polarized. In other words, the two regions wouldshare components in one direction. The amount that the polarization ofthe second portion 25 of the beam is rotated in retarder 20 affects howmuch of such shared component will be found in the altered beam.

A polarization-selective element in the form of a polarizing filter 28,also referred to as a polarizer, is disposed downstream from thepolarization-altering element 20 along the optical axis. Polarizer 28has a transmission axis transverse to optical axis 12. The polarizer isarranged to transmit light having a polarization direction parallel tothe transmission axis but to block transmission of light having apolarization direction orthogonal to the transmission axis. The lightfrom altered beam 24 passing through polarizer 28 forms an output beam30. The output beam, like the source beam, is focused at nominal focallocation 18.

Depending upon the relationship between the polarization directions ofthe beam portions in altered beam 24 and the transmission axis ofpolarizer 28, the polarizer will pass or block light in the first andsecond portions of the altered beam. Thus, if the transmission axis ofpolarizer 28 is aligned with the polarization direction of the firstbeam portion (from hole 22) and if the polarization direction of thesecond beam portion (from retarder 20) is perpendicular to thetransmission axis, the output beam will consist essentially of lightfrom the first portion. In this condition, the output beam 30 will havea high effective f-number; its characteristics will resemble those of abeam which would have been produced by a small aperture in an opaqueplate. The output beam will have a good depth of field, but a relativelylarge spot size at the nominal focal location. The relationship of spotsize versus distance along the optical axis for such a beam is indicatedby curve 32 in FIG. 2.

If the transmission axis of the polarizer is aligned with thepolarization direction of the second beam, the output beam 30 will havea substantially annular intensity distribution, and a low effectivef-number.

A scanner according to a further embodiment of the invention includes aframe 50, illumination apparatus 52 incorporating the features discussedabove with reference to FIG. 1 arranged to provide an output beam 30 inthe manner discussed above. The scanner further includes a moving ortime-varying optical element such as a moving mirror 54 disposed alongthe optical axis 12 and driven by a conventional actuator between theposition illustrated in solid lines and the position shown in brokenlines at 54′ for moving the output beam and optical axis so as to sweepthe focal location 18 in a pattern within a working region 51. Any otherconventional moving or time-varying optical element, such as a movingprism or moving holographic element, can be employed for this purpose.Also, although the moving element is depicted as downstream from theentire illumination apparatus 52, the moving element may be disposedupstream from the polarizer. Alternatively, the entire illuminationapparatus 52 can be moved to move the focal location. The scannerfurther includes a conventional photodetector 58 for detecting lightreflected from the working region and providing a signal representingsuch light. In the particular embodiment of FIG. 3, photodetector 58 hasa broad field of view which is fixed relative to the frame. In otherscanners, commonly referred to as retro-reflective or “retro” scanners,the light reflected from the objects being scanned is directed by thesame moving optical elements used to sweep the spot from the lightsource. The photodetector views the objects being scanned through themoving optical elements, and the field of view of the scanner moves withthe spot from the illumination source. Either arrangement may beemployed. In a further alternative, the information-bearing elements tobe scanned can be moved relative to the frame. For example, a conveyormay move objects past a fixed frame, or else the frame may be in theform of a pen which can be moved manually. In the foregoingarrangements, the elements of the system which are used to move theinformation-bearing element relative to the beam or spot from theillumination system act as means for sweeping the output beam across thelight and dark areas of the indicia.

A scanner according to these embodiments of the invention can be used inthe conventional manner. A low f-number output beam is preferred forscanning high-resolution (small feature size) bar codes but requiresthat the bar coded object be positioned in a small range of locationsrelative to the scanner. A high f-number output beam is preferred forscanning low-resolution (large feature size) bar codes at varyingdistances from the scanner.

The elements depicted in FIG. 1 are drawn to near scale, except that theaxial distance between the lens and focal location or nominal beam waistlocation 18 is shortened. In a typical system with the lensapproximately 4.5 mm downstream from the VLD, the beam waist locationmay be 250 mm downstream from the lens.

The apparatus discussed above can be arranged to provide a non-circularspot. The apparatus depicted in FIG. 4 is generally similar to theapparatus discussed above in connection with FIG. 1. Here again, thelight source 310 produces a source beam of light 313 which isplane-polarized and focused at a focal location 318 along the opticalaxis 312. However, in the apparatus of FIG. 4, the hole 322 in theretarder or birefringent element 320 is elongated in a direction ofelongation E transverse to the optical axis 312. Accordingly, the firstportion 323 of the altered light beam 324 passing through the hole 322in the retarder has an elongated cross-sectional shape, with its longaxis in the same direction E.

Here again, the light in the first portion 323 of the altered beam,immediately surrounding axis 312, has a first polarization directionwhereas the light in the second portion of the beam has a secondpolarization direction different from the first polarization direction.However, in the embodiment of FIG. 4, the first portion of the beam hasa non-circular cross-section corresponding to the noncircularcross-section of hole 322. If the transmission axis of the polarizer orfilter 328 is parallel to the first polarization direction, the filterwill block the light having the second polarization direction, so thatthe output beam 330 will consist essentially of the light in the firstportion 323 of the altered beam. The beam will have a large effectivef-number and will form a relatively large spot 335 at the nominal focallocation with relatively good depth of field. This spot will have anon-circular shape corresponding to the non-circular shape of hole 322.Preferably, the smaller dimension of the spot is aligned with thedirection of motion imparted by the movable optical elements of thescanning apparatus or other scanning arrangements.

If the transmission axis of the polarizer or filter 328 is parallel tothe second polarization direction, the output beam will consistessentially of the light in the second portion 325 of the altered beam,and hence the beam will form a different spot 331 at the nominal focallocation. If the polarization-altering element 320 is disabled, theentire altered beam will have the same polarization direction. If thetransmission axis of the polarizer 328 is aligned with this polarizationdirection, essentially all of the light in the source beam will beincluded in the output beam. The size and shape of this spot aredetermined in part by the size and shape of the source beam 313 providedby the light source 310. For example, a light source incorporating alaser diode and lens may provide a noncircular source beam 313, whichmay yield a smaller spot 331, at the nominal focal location, having anoncircular outer border corresponding to the shape of the original beamand having a lower effective f-number.

Illumination apparatus according to a further embodiment isschematically shown in FIG. 5. This apparatus incorporates a lightsource 110, polarization-altering retarder 120 andpolarization-selective filter 128 similar to the corresponding elementsof the embodiment discussed above with reference to FIG. 1. Each ofthese elements is movable relative to the frame 140 of the apparatus.Actuators 142, 144 and 146 are provided for moving these elementsrelative to optical axis 112 and frame 140. Actuators 142, 144 and 146may be conventional mechanical or electromechanical actuators of anyconvenient type, as, for example, motion driven linkages, pneumaticcylinders, solenoids or the like. Operation of actuator 142 will varythe polarization direction of the source beam; operation of actuator 144will change the orientation of retarder 120; whereas operation ofactuator 146 will vary the orientation of the transmission axis ofpolarizer 128. Any one of these variations can be used to change therelationship between the polarization directions of the beam portions inaltered beam 124 and the transmission axis of the polarizer. This willvary the shape of the output beam. In practice, of course, it is notnecessary to move all of the elements, and only one actuator need beprovided for moving any one of the source; the retarder orpolarization-altering element; or the polarizing filter orpolarization-selective element.

If the apparatus is arranged to turn the retarder orpolarization-altering element 120, the angle between the polarization ofthe source beam and the fast direction of the retarder can be altered toenable and disable the polarization-altering element. When the fast axisof the retarder is aligned with the polarization direction of the sourcebeam, the retarder does not alter polarization of the source beam andhence the retarder is effectively disabled. If the polarizationdirection of the source beam is aligned with the transmission axis ofthe polarizer, the output beam will include all of the light in thesource beam when the polarization-altering element is disabled. In thiscondition, the output beam will resemble a beam formed by alarger-diameter aperture. The output beam will have a low effectivef-number and a small spot size at nominal focal location 18, butrelatively poor depth of field. The relationship of spot size versusdistance along the optical axis for such a beam is indicated by curve 34in FIG. 2. When the polarization-altering element is enabled and rotatesthe polarization direction 90°, the output beam will include only thelight in the first beam portion (passing through the hole in theretarder). In this condition, the output beam will have a high effectivef-number and relatively large spot size at the nominal focal location,but will have good depth of field. The relationship between spot sizeand distance along the optical axis will be as indicated by curve 32 inFIG. 2. Apparatus with time-varying beam characteristics can beincorporated in a scanner as discussed above.

In a particularly preferred method of operation, the actuator oractuators is driven to repeatedly change the characteristics of theoutput beam in the scanner between a large effective f-number (curve 32,FIG. 2) and a small effective f-number (curve 34, FIG. 2). For example,the actuator or actuators may be driven by a periodic driving signal soas to vary the beam characteristics periodically. Desirably, thisvariation occurs rapidly, so that as the scanner attempts to decipher abar code on a given object, the bar code will be scanned by first by abeam having one f-number and then by a beam having another f-number. Ina typical scanner which performs tens of scans per second, the f-numberof the output beam may be varied at a rate of tens of Hz. Variation inthe output beam f-number may be synchronized with the scanning motion ofthe beam, so that the beam sweeps through one or more complete scans atone f-number and then changes to another f-number and holds thatf-number through one or more scans. If the object has a relativelylow-resolution bar code, with large feature size, but is disposed at asubstantial distance from the nominal focal location, the bar code willbe successfully read by the beam having the high f-number (curve 32). Ifthe object has a small-feature bar code, but is properly positioned atthe nominal focus, the bar code will be successfully read by the lowf-number beam (curve 34). The system will provide performance matchingthat which would be achieved by using a light source having a curve ofspot size versus distance indicated by composite curve 40 in FIG. 2, andwill provide better overall performance than that which could beachieved using a fixed beam configuration.

In another variant, the frequency of the variation in f-number may besubstantially higher than the scanning frequency and higher than theexpected frequency of light-to-dark intensity transitions encountered asthe beam sweeps across the information bearing object, so that the beamchanges f-number repeatedly during each complete scan. The signal fromthe photodetector which receives the light from the scanned object canbe sampled in synchronism with the change in f-number using conventionalanalog or digital sampling techniques, and the samples taken atdifferent f-numbers can be separated into two separate series. Eachseries represents the results of a scan at one f-number, and can beprocessed in the same manner as samples from a conventional scan todecipher the bar code.

The retarder or birefringent element need not have a region physicallyremoved from it. It is sufficient that it have two regions withdifferent rotation properties. In the embodiments discussed above withreference to FIGS. 1-5, the hole in the retarder provides a regionhaving zero rotation whereas the remainder of the retarder has a finiterotation. In other embodiments, different portions of the retarder mayprovide different non-zero rotations. For example, the retarder 220 ofFIG. 6 has regions of different thickness to provide different degreesof rotation. Also, the retarder may be made from different materials indifferent regions, or subjected to different conditions which influenceits degree of birefringence unequally.

The illumination apparatus of FIG. 7 includes a light source 410 andpolarization-selective element 428 similar to those discussed above. Thepolarization-altering element 420 includes a transparent container 430holding a material such as a nematic liquid crystal havingpolarization-rotating properties dependent upon the magnitude of anelectric field applied across the material. A first pair of transparentelectrodes 422 a and 422 b cover opposite faces of the container in acentral region adjacent the optical axis 412. These electrodes areconnected to a first variable-voltage source 424. A second pair oftransparent electrodes 426 a and 426 b cover a peripheral region of thecontainer, remote from the optical axis. The second pair of electrodesis connected to a second variable voltage source 428. By varying thevoltages applied to either or both pairs of electrodes, thepolarization-altering properties of element 420 can be varied in thecentral region, in the peripheral region, or both.

Apparatus according to a further embodiment of the invention (FIG. 8)includes a source 510, polarization-altering element 520 andpolarization-selective element 528 similar to the corresponding elementsdiscussed above with reference to FIG. 1. A movable optical element suchas a mirror 522 can be selectively interposed along the path of the beamas indicated at 522′ and 526′ so as to selectively divert the beamaround the polarization-selective element along an alternate pathdefined by mirrors 524, 525 and 526 and thereby disable thepolarization-selective element 528. In effect, the optical axis is movedrelative to the polarization-selective element 528, while leaving thepolarization-selective element in fixed position relative to the frameof the apparatus. A similar arrangement can be used to divert the beamaround the polarization-altering element 520 so as to disable or enablethat element. In another arrangement, the polarization-altering elementcan be selectively disabled by moving an auxiliary polarization-alteringelement into and out of the beam path. For example, where thepolarization-altering element is arranged to rotate the polarizationdirection of one portion of the beam by 90°, the auxiliarypolarization-altering element can rotate the polarization direction ofthe same portion of the beam by 90°. When the auxiliarypolarization-altering element is operative, the total rotation is 180°or 0°.

Both the distribution and intensity of the light determines the shape ofthe beam downstream. The beam may include more than two portions withdifferent polarizations. Provided that the various portions of the beamhave a plane of symmetry coincident with the optical axis, the outputbeam will also have a plane of symmetry coincident with the opticalaxis. The invention also can be applied to production of other beamshapes. For example, if one portion of the altered beam has a firstpolarization has an intensity distribution predominantly on one side ofthe optical axis, and another portion of the altered beam has anintensity distribution on the other side of the optical axis, the outputbeam can be shifted relative to the optical axis by varying the actionof the polarization selective element.

The embodiments discussed above take advantage of the inherent linearpolarization of light from a VLD to manipulate the beam shape. Whilelaser light typically is monochromatic and coherent, it does notnecessarily have one particular polarization. VLD's typically producelinearly polarized light; certain gas lasers do not. Therefore, if anunpolarized He-Ne laser or other unpolarized light emitter is used inthe light source, the light source should include apolarization-selective element downstream from the emitter but upstreamfrom the polarization-altering element.

In the embodiments discussed above, a lens is used to focus the sourcebeam. However, other optical elements such as mirrors and holographicoptical elements can be used instead. Also, additional focusing elementssuch as lens, mirrors or holograms can be positioned anywhere along theoptical axis. Further, the optical axis typically is not a straightline; elements such as mirrors and prisms can be employed to fold theoptical axis. A moving mirror, prism or other moving element, such aselement 54 (FIG. 3), can act both to fold the optical axis and to sweepthe beam.

In the discussion above, the polarizer is referred to as either blockingor transmitting light. An ideal polarizer, operating on perfectlypolarized light, would transmit all of the incident light having apolarization direction aligned with the transmission axis, and wouldtransmit none of the light having a polarization direction perpendicularto the transmission axis. Real polarizers are not perfect. Thus, a realpolarizer will transmit some small fraction of the light having apolarization direction perpendicular to the transmission axis and willattenuate light having a polarization direction parallel to thetransmission axis. Also, the light in a given portion of the beamtypically is not perfectly polarized, and its polarization direction maynot be perfectly parallel or perfectly perpendicular to the transmissionaxis of the polarizer. For these reasons as well, unintended attenuationor transmission may occur. References in this disclosure to a polarizerblocking or transmitting light should be understood in this context asnot requiring perfect blocking or transmission. However, the intendedeffect (blocking or transmitting light) should predominate. For example,where the polarizer is intended to block transmission of light in aparticular portion of a beam, the amount of such light which istransmitted should be substantially smaller than the amount of suchlight which is blocked by the polarizer. Also, thepolarization-selective element need not absorb the light which itblocks. For example, the polarization-selective element may be arrangedto reflect or diffract the blocked light, so that it does not passdownstream into the output beam.

As used in this disclosure, the term “light” should be understood asincluding electromagnetic radiation in the infrared and ultravioletregions of the spectrum, as well as visible light.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

What is claimed is:
 1. A scanner for reading indicia having light and dark areas comprising: (1) illumination apparatus for forming an output beam, the illumination apparatus including: (a) a light source adapted to emit a source beam of linearly-polarized light in a downstream direction along an optical axis, said source beam having a polarization direction transverse to said axis; (b) a fixed birefringent plate that intercepts only a portion of the source beam, said birefringent plate having a fast axis transverse to said optical axis so that the polarization of said source beam is altered only in that portion of the source beam which is intercepted by said birefringent plate so as to form an altered beam having a first portion with a first polarization direction and a second portion with a second polarization direction transverse to said first polarization direction, said first portion having a first intensity distribution relative to said axis, said second portion having a second intensity distribution relative to said axis; (c) at least one polarization-selective element mounted downstream from said birefringent plate, at least one of said polarization-selective element being operative to allow transmission of light having one polarization direction and to prevent transmission of light having another polarization direction, (2) means for sweeping the output beam across the indicia so that the output beam impinges on the light and dark areas of the indicia at different times during such sweeping and so that light reflected from the indicia will have amplitude which varies depending on whether the beam is impinging on a light area or a dark area; and (3) a photodetector for receiving the light reflected from the indicia and producing a signal representing the amplitude of said reflected light, whereby said signal will vary in accordance with said light and dark areas of said indicia.
 2. A scanner as claimed in claim 1 wherein said first and second intensity distributions are different from one another.
 3. A scanner as claimed in claim 1 wherein said birefringent plate is arranged to provide said intensity distributions having a plane of symmetry.
 4. A scanner as claimed in claim 1 wherein said first intensity distribution substantially excludes light outside of a central zone close to said axis whereas said second intensity distribution includes light outside of said central zone.
 5. A scanner as claimed in claim 4 wherein said source includes an optical element focusing said source beam to a focal location, said birefringent plate and said at least one polarization-selective element being disposed between said optical element and said focal location, whereby said birefringent plate, and said at least one polarization-selective element determine the f-number of said output beam.
 6. A scanner as claimed in claim 1 wherein said birefringent plate provides approximately 90° difference between the polarization directions of said first and second portions of said altered beam.
 7. A scanner as claimed in claim 1 wherein said source includes a laser diode.
 8. A scanner as claimed in claim 7 wherein said source further includes a lens downstream of said laser diode, said lens focusing light from said laser diode at a focal location on said axis.
 9. A scanner as claimed in claim 1 wherein said birefringent plate has a hole extending through it, said hole encompassing said axis, whereby said first portion of said altered beam will include light passing through said hole whereas said second portion of said altered beam will include light passing through said birefringent plate.
 10. A scanner as claimed in claim 1 further comprising a frame, said illumination apparatus being mounted to said frame, said means for sweeping the output beam including a moveable or variable optical element for moving said output beam relative to said frame.
 11. A scanner for reading indicia having light and dark areas comprising: (1) illumination apparatus for forming an output beam including: (a) a support structure; (b) a light source including a laser diode and a lens mounted to said support structure for directing a source beam light in a downstream direction along an optical axis and focusing the source beam at a focal location on said optical axis, the light of the source beam having a polarization direction transverse to the axis; (c) a birefringent plate having a hole therein mounted to said support structure so that said hole is aligned with said axis and the fast axis of the birefringent plate is transverse to the optical axis, said hole being sized so that a peripheral portion of said source beam impinges on said birefringent plate around said hole whereas a central portion of said source beam passes through said hole; and (d) a polarizer mounted to said support structure downstream from said birefringent plate, said polarizer having a transmission axis transverse to said axis; (2) means for sweeping the output beam across the indicia so that the output beam impinges on the light and dark areas of the indicia at different times during such sweeping and so that light reflected from the indicia will have amplitude which varies depending on whether the beam is impinging on a light area or a dark area; and (3) a photodetector for receiving the light reflected from the indicia and producing a signal representing the amplitude of said reflected light, whereby said signal will vary in accordance with said light and dark areas of said indicia.
 12. A scanner as claimed in claim 11 wherein said birefringent plate is a half-wave plate.
 13. A scanner as claimed in claim 12 wherein the birefringent plate has a fast axis transverse to said optical axis, said fast axis being disposed at approximately 45° to the polarization direction of the source beam.
 14. A scanner as claimed in claim 11 wherein said birefringent plate has a fast axis transverse to the optical axis and said birefringent plate is rotatable relative to said support structure so as to vary the angular relationship between the fast axis of the birefringent plate and the polarization direction of the source beam.
 15. A scanner as claimed in claim 11 wherein said laser diode is rotatable relative to said support structure so as to vary the angular relationship between the fast axis of the birefringent plate and the polarization direction of the source beam.
 16. A scanner as claimed in claim 11 wherein said polarizer is rotatable relative to said support structure so as to vary the relationship between the transmission axis and the polarization direction of the source beam.
 17. A scanner as claimed in claim 11 further comprising said sweeping means including means for moving said output beam relative to said support structure.
 18. A method of scanning information-bearing elements comprising: (1) providing an output light beam by: (a) directing a source beam of linearly-polarized light in a downstream direction along an optical axis; (b) nonuniformly altering the polarization direction of said source beam by intercepting only a portion of said source beam with a birefringent plate to provide an altered beam having a first portion with a first polarization direction and a second portion with a second polarization direction transverse to said first polarization direction, said first portion having a first intensity distribution relative to said axis, said second portion having a second intensity distribution relative to said axis; and (c) passing the altered beam through a fixed polarization-selective element having a transmission axis so as to allow light having a polarization direction matching the transmission axis of said fixed polarization-selective element to pass downstream into an output beam and substantially exclude components with polarization directions perpendicular to the transmission axis of said fixed polarization-selective element from said output beam; (2) moving said output beam relative to the information-bearing elements so that the output beam passes over light and dark regions of the information-bearing elements at different times, and (3) detecting light reflected by the information-bearing elements so as to produce a signal representing the light and dark regions.
 19. A method as claimed in claim 18 wherein said first and second portions of said altered beam have planes of symmetry at said transmission axis.
 20. A method as claimed in claim 18 wherein said step of directing a source beam includes focusing said source beam at a focal location on said optical axis, whereby said output beam will be focused at said focal location, and wherein said steps of altering polarization and passing the altered beam through said polarization-selective element determine the spot size and depth of field of the focused output beam.
 21. A method as claimed in claim 20 wherein said first portion of said altered beam substantially excludes light outside of a central zone close to said axis whereas said second portion of said altered beam includes light outside of said central zone.
 22. A method as claimed in claim 21 wherein said polarization-selective element substantially blocks transmission of one of said portions into said output beam.
 23. A method as claimed in claim 20 wherein said spot size and depth of field are varied periodically.
 24. Illumination apparatus for forming an output beam, the apparatus comprising: (a) a light source adapted to emit a source beam of linearly-polarized light in a downstream direction along an optical axis, said source beam having a polarization direction transverse to said axis; (b) a fixed birefringent plate that intercepts only a portion of the source beam, said birefringent plate having a fast axis transverse to said optical axis so that the polarization of said source beam is altered only in that portion of the source beam which is intercepted by said birefringent plate so as to form an altered beam having a first portion with a first polarization direction and a second portion with a second polarization direction transverse to said first polarization direction, said first portion having a first intensity distribution relative to said axis, said second portion having a second intensity distribution relative to said axis; (c) at least one polarization-selective element mounted downstream from said birefringent plate, at least one of said polarization-selective element being operative to allow transmission of light having one polarization direction and to prevent transmission of light having another polarization direction; wherein said source includes a laser diode; wherein said source further includes a lens downstream of said laser diode, said lens focusing light from said laser diode at a focal location on said axis; and wherein said birefringent plate abuts said lens.
 25. Apparatus as claimed in claim 24 further comprising a source housing, said lens and said birefringent plate being mounted to said housing. 